The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a polishing pad having a grooved pattern for a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photoresist layer placed thereon is also non-planar. A photoresist layer is typically patterned by a photolithographic apparatus that focuses a light image onto the photoresist. If the outer surface of the substrate is sufficiently non-planar, the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus. Then it will be impossible to properly focus the light image onto the entire outer surface. Therefore, there is a need to periodically planarize the substrate surface to provide a flat surface for photolithography.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface.
A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be supplied to the polishing pad to provide an abrasive chemical solution at the interface between the pad and the substrate. CMP is a fairly complex process, and it differs from simple wet sanding. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and abrasive particles with the reactive sites on the substrate results in polishing.
An effective CMP process has a high polishing rate and generates a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. The polishing rate sets the time needed to polish a layer. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing time needed to achieve the required finish and flatness sets the maximum throughput of the CMP apparatus.
One problem in CMP relates to slurry distribution. As was indicated above, the CMP process is fairly complex, requiring the interaction of the polishing pad, abrasive particles and reactive agent with the substrate to obtain the desired polishing results. Accordingly, ineffective distribution of the slurry across the surface of the polishing pad provide less than optimal polishing results. Polishing pads have been used which include perforations about the pad. The perforations, when filled, distribute slurry in their respective local region as the polishing pad is compressed. This method of slurry distribution has limited effectiveness because each perforation in effect acts independently. Thus, some of the perforations may have too little slurry, while others may have too much slurry. Furthermore, there is no way to directly channel the excess slurry to where it is needed.
Another problem in CMP is "glazing" of the polishing pad. Glazing occurs when the polishing pad is heated and compressed in regions where the substrate is pressed against it. The peaks of the polishing pad are pressed down and the pits of the polishing pad are filled up, so the surface of the polishing pad becomes smoother and less abrasive. As a result, the polishing time required to polish a substrate increases. Therefore, the polishing pad surface must be periodically returned to an abrasive condition, or "conditioned", to maintain a high throughput.
In addition, during the conditioning process, waste materials associated with abrading the surface of the pad may fill or clog the perforations in the polishing pad. Filled or clogged perforations can not hold slurry, thereby reducing the effectiveness of the polishing process.
An additional problem associated with filled or clogged perforations relates to the separation of the polishing pad from the substrate after polishing has been completed. The polishing process produces a high degree of surface tension between the polishing pad and the substrate. The perforations decrease the surface tension by reducing the contact area between the polishing pad and the substrate. However, as the perforations become filled or clogged with waste material, the surface tension increases, making it more difficult to separate the polishing pad and the substrate. As such, the substrate is more likely to be damaged during the separation process.
Yet another problem in CMP is referred to as the "planarizing effect". Ideally, a polishing pad only polishes peaks in the topography of the substrate. After a predefined period of polishing, the areas of these peaks will eventually be level with the valleys, resulting in a planar surface. However, if a substrate is subjected to the "planarizing effect", the peaks and valleys will be polished simultaneously. The "planarizing effect" results from the compressible nature of the polishing pad in response to point loading. In particular, if the polishing pad is too flexible, it will deform and contact a large surface area of the substrate.
Accordingly, it would be useful to provide a CMP system which reduces or solves some, if not all, of these problems.